High precision stages have been extensively used in many machining and manufacturing facilities such as semiconductor processing, machine tool metrology and assembly line testing because those stages typically have high positioning accuracy with a large dynamic range and a wide bandwidth. Here, the displacement sensing technology plays a key role for the fast and robust positioning control and high resolution measurement. Displacement is one of the most fundamental quantities for precision stage applications. Non-contact sensors such as capacitive displacement sensor and optical sensor, laser interferometer, laser encoder and position sensitive detector (PSD), have been commonly used in high precision stage applications because those are acceptable for dynamic motion characterization and fast and high resolution measurement. However, up to recently, the capacitive displacement sensor and PSD have been more preferred to the laser interferometer and laser encoder because they are so compact that they can be easily embedded into the stage.
In a current displacement measurement technology for precision stage applications, the capacitive type sensors have been mostly widely used because these can detect motion at sub-nanometer levels directly and provide accuracy, linearity, resolution, and stability. But the target must be conductive, and attractive force between the target and the sensor probe should be canceled. Unfortunately, they are relatively expensive. On the other hand, PSDs are commonly used because these are cheap and capable of measuring lateral displacement in one or two dimensions. They consist of two, four or more segmented photodiodes positioned symmetrically around the center of the detector and separated by a narrow gap and anode and common cathode contacts as seen in FIG. 1. The position information is derived from the optical signal powers received by the segmented each photodiode and defined by the relative position of the beam spot with respect to the center of the devices. However, many issues about the position accuracy and resolution have been raised due to the gap size between the elements, incident light intensity uniformity and aberration, the optical alignment and the doping uniformity of the each active area. Moreover, the sensitivity issue of PSD is crucial. It is well-known that the smaller the beam spot size, the higher the sensitivity. It is thus effective to make the beam spot small as possible to improve the sensor sensitivity. However, the minimum beam spot size is limited because the gaps exist between each photodiode, which are not sensitive to the beam spot.
Many works on the nanometer resolution displacement sensing instrumentation that can be implemented to high precision stages have been performed since a few decades ago. The companies, Physik Instrumente, Lion Precision and InSitu Tec, have been using the capacitive sensors to be integrated with flexure stages, and achieved sub-nanometer resolution. Tang first introduced the lateral comb drive in an electrostatic resonator, and has since been used numerous times in both actuator and sensing configurations. Yong presented a novel piezoelectric strain sensor for a high speed nanopositioning stage, and Bazaei reported a novel piezoresistive sensor embedded with micro-electromechanical system-based (MEMS) nanopositioner. Zhu introduced an electrothermal position sensor consisted of two beam-shaped resistive heaters made from single crystal silicon for a micromachined nanopositioner. Parmar used a laser encoder to test the tracking performance for a large range single-axis nanopositioning system based on a moving magnetic actuator and a flexure stage. Lee reported a PSD-based multi-axis displacement measurement sensor for nanopositioning stage and Gao introduced a high sensitivity optical displacement sensor based on single photodiode. However, simple, cheap, noncontact and high resolution sensor that can be easily embedded into a high precision stage has not been well-documented.